Yes I just mentioned lib2to3 in the post, that's probably the closest example. (There are also some Clang-based tools to convert C++ 03 to 11 to 14, etc.)

Although I believe that you need different terminology. A concrete syntax tree doesn't necessarily represent comments and whitespace. It represents the derivation of the string from the grammar. These are totally different things and I give an example in the article.

The CST is kind of wasteful and I'm not sure why you would use it other than not wanting to write semantic actions to prune the tree on the fly. If anyone has insight into this, please let me know.